The present invention relates to a laser cavity, and in particular to a ring laser cavity.
Laser cavities configured as rings have been widely used to eliminate a problem known as spatial hole burning. Spatial hole burning occurs in laser cavities which allow light to propagate in opposing directions within a gain medium, for example in a linear laser cavity. Light in such a laser cavity forms a standing wave, which selectively depletes the gain medium according to the localised intensity distribution of this standing wave (spatial hole burning). The effect of the spatial hole burning is to make several longitudinal modes of the laser cavity oscillate simultaneously in a homogeneously broadened laser (which may otherwise exhibit single longitudinal mode behaviour), thereby compromising the spectral purity of the laser. This multimode behaviour can cause instabilities in laser systems that are intra-cavity doubled (T. Baer, ‘Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers,’ J. Opt. Soc. Azm B 3, 1175 (1986)).
In a ring laser, light generated by stimulated emission in the gain medium is made to propagate in one direction only around the ring, thereby preventing the formation of standing waves and avoiding the resulting spatial hole burning in the gain medium. Since there is no spatial hole burning in the gain medium, the laser may oscillate in a single longitudinal mode (provided that the gain medium is predominantly homogeneously broadened). The single longitudinal mode in an intracavity doubled, uni-directional ring laser is considerably more stable than the multiple modes seen in a conventional intra-cavity doubled laser cavity, and is significantly less sensitive to environmental changes, such as temperature fluctuations.
If light is allowed to propagate in both directions in a ring cavity, spatial hole burning can occur and the main advantages of a ring laser cavity over a linear laser cavity are lost.
In order to allow the light to propagate in only one direction around a ring laser cavity, the loss suffered by light propagating in an unwanted direction must be made greater than the loss suffered by light propagating in the preferred direction. Provided that the difference between the two losses is sufficient, the laser cavity will oscillate in the preferred direction only.